Method for manufacturing a piezoelectric transformer device

Abstract

The invention relates to a method for manufacturing a piezoelectric transformer device (100). In a first step, a first layer (11) of silver chromium alloy is applied to an end face (10) of a first piezo disk (1) and, in a second step, a second layer (12) of metal or metal alloy having a lower melting point is applied as a silver alloy. The first piezo disk (1) and a second piezo disk (2) are arranged such that the end face (10) of the first piezo disk (1) is adjacent the second piezo disk (2). In a connecting step, the first piezo disk (1) and the second piezo disk (2) are baked together by means of liquid phase diffusion under the action of pressure and temperature. The invention further relates to a correspondingly manufactured transformer device (100) and to a device for determining and / or monitoring a process variable.

Description

[0001] Method for manufacturing a piezoelectric transducer device

[0002] The invention relates to a method for manufacturing a piezoelectric transducer device. Furthermore, the invention relates to a correspondingly manufactured transducer device and a device for determining and / or monitoring at least one process parameter. The process parameter is, for example, the fill level, density, viscosity, or sugar content of a medium such as a liquid or bulk material.

[0003] It is known in the art to use vibration sensors to detect whether liquids or bulk solids have reached or fallen below a limit level. Such sensors also allow, for example, the measurement of density or viscosity as process variables of the medium to be determined. During the measurements, a mechanically vibrating unit is excited to mechanical vibrations, and the process variables are derived from the received mechanical vibrations, which depend on an interaction with the medium. Transducer devices are used to generate the vibrations, to receive them, and to make them available as electrical signals. These transducer devices generate the mechanical vibrations from excitation signals and convert them into received electrical signals. It is known in the art to use piezoelectric elements in the transducer devices.It is common practice to combine several piezoelectric discs into a compact unit. This is achieved, for example, through silver diffusion under the influence of pressure and temperature on the piezoelectric elements. However, the long manufacturing time has proven to be a disadvantage. The bonding process can sometimes take between 10 and 20 hours. It is crucial that the individual piezoceramic discs are fully fused together. This can be achieved by welding at a temperature significantly above the Curie temperature of the piezoceramic, for example, at 700 °C to 800 °C. This means that the polarization is only generated after welding. The objective of the invention is to propose an acceleration of the manufacturing of transducer devices.In particular, the manufacturing process should take place at temperatures below the Curie temperature of the ceramic used, so that preferably no step of generating polarization after diffusion welding is required.

[0004] The invention solves the problem by a method for manufacturing a piezoelectric transducer device, wherein the method comprises at least the following steps: that on at least one end face of at least one first piezo disk, in a first step a first layer of silver alloy and in a second step a second layer of metal or metal alloy is applied to the first layer.which differs from the silver alloy and has a lower melting point than the silver alloy, is applied, wherein a silver-chromium alloy is applied as the silver alloy, in that at least the first piezo disk and a second piezo disk are arranged relative to each other in such a way that the end face of the first piezo disk is adjacent to the second piezo disk, and in a joining step at least the first piezo disk and the second piezo disk are baked together under the influence of pressure and temperature by means of liquid phase diffusion.

[0005] According to the invention, the at least two piezoelectric discs are joined together by liquid-phase diffusion. An alternative term for liquid-phase diffusion is liquid-phase sintering. This occurs through the application of temperature and pressure to the piezoelectric discs. The pressure acts in such a way that the piezoelectric discs are pressed together. The special feature is that not only is a silver alloy in the form of a silver-chromium alloy applied to the end face of a piezoelectric disc, but the joining process can be accelerated by the additional metal or metal alloy. The additional metal or metal alloy differs from the silver alloy and, in particular, has a lower melting point than the silver alloy.

[0006] In general terms, it can also be stated that a first metal (preferably a precious metal, and in the following example, silver) or a corresponding metal alloy (i.e., an alloy with a precious metal, e.g., a silver alloy) is applied to the front face. A layer of a second—different—metal or a second metal alloy is then applied to this. The second metal (or the second metal alloy) has a lower melting point than the first metal.

[0007] Regarding the application method, a distinction can be made between two variants:

[0008] One variant involves applying a first layer of silver alloy to at least one end face of at least one piezoelectric disc. In a second step, a second layer of a metal or metal alloy, differing from the silver alloy and having a lower melting point, is applied over the first layer. Thus, two layers are applied, and during the bonding process, the metal or metal alloy with the lower melting point diffuses into the metal lattice of the metal or metal alloy with the higher melting point (in this case, the silver alloy). The resulting intermetallic phase forms a bond that remains stable well above the bonding temperature.

[0009] An alternative approach involves applying a first layer of silver-metal alloy to at least one end face of at least one piezoelectric disc in a first step. In this approach, an alloy of the aforementioned substances—that is, a silver alloy and a different metal with a lower melting point—is already applied to the end face. A sputtering process or electroplating, for example, is suitable for this.

[0010] In both cases, the resulting material combination leads to a significantly faster connection between the piezoelectric discs. It is advantageous if silver is present at a concentration of between 75% and 85% by mass, and tin, as an example of the added metal with a lower melting point, is present at a concentration of between 15% and 25% by mass. Preferably, a configuration contains 80% by mass of silver and 20% by mass of tin. When two piezoelectric discs are mentioned here and in the following, this may include multiple discs, depending on the specific design. Furthermore, when referring to what happens to a first piezoelectric disc, it is implied that subsequent piezoelectric discs are treated in the same way.

[0011] One embodiment of the process involves applying the first layer and / or the second layer by means of sputtering. Sputtering, or "sputter deposition," is a known method for coating a substrate with a material that has been previously atomized.

[0012] One embodiment of the process involves applying tin as the metal or a tin alloy as the metal. In this embodiment, the first layer of a silver alloy is covered with a layer of tin or a tin alloy. This results in a silver-tin alloy when the piezoelectric discs are bonded together.

[0013] Alternatively, the following metals or metal alloys are used: bismuth, with the eutectic point with silver at approximately 250 °C; tin and bismuth, eutectic point at 145 °C; zinc and bismuth with a eutectic point at 255 °C; zinc and lead with a eutectic point at 180 °C; or, for example, bismuth and lead with a eutectic point at 130 °C.

[0014] Another embodiment involves applying indium as the metal or an indium alloy as the metal. The purpose of the indium is to further reduce the temperature during the joining process.

[0015] A supplementary embodiment of the process provides for the application of a tin-indium alloy as the metal alloy.

[0016] One embodiment of the process involves applying a third layer of silver or a silver alloy to the second layer in a third step. In this embodiment, the second layer of metal or metal alloy is covered by a third layer of silver or a silver alloy. This prevents, for example, the oxidation of the second layer. Since, for instance, tin oxidizes easily in air and an oxide layer interferes with diffusion welding, the tin layer, for example, is covered with silver, gold, or another precious metal (preferably with a sufficient layer thickness of 0.2–0.5 pm).

[0017] In one embodiment, the third layer is approximately half the height of the first layer. In a supplementary embodiment, the mass ratios of 75% to 85% silver and 25% to 15% tin are maintained.

[0018] One embodiment of the process provides that, in a further step, metal or a metal alloy – and preferably silver or a silver alloy – is applied to the third layer. In this embodiment, at least one further layer is applied. Additional layers can also be applied, with metal / metal alloy and silver / silver alloy alternating.

[0019] One embodiment of the process involves maintaining a temperature acting on at least the first and second piezoelectric elements above a solidus line of the metal or metal alloy with the lower melting point for a predetermined period during the joining step. The solidus line is the boundary in the phase diagram of metals or alloys that separates the region with only solid phases from the region with both solid and liquid phases. In a pure metal, the solidus temperature coincides with the liquidus temperature.

[0020] Corresponding values ​​are well known for the different materials. In one embodiment, the temperature during the joining step lies between 0.5 °C and 10 °C above the temperature of the solidus line.

[0021] One embodiment of the method involves aging at least the first and second piezoelectric elements in an aging step during the connection process by introducing elevated temperatures. In this embodiment, the piezoelectric elements are subjected to elevated temperatures so that they are already aged before use. This prevents settling or aging effects in the installed state, e.g., in a vibronic sensor, and before its use, e.g., in a process plant. It also stabilizes the piezoelectric material for the operating temperatures. The aging temperatures preferably correspond to the temperature range in which the transducer device is intended to be used. In particular, the temperatures are below the Curie temperature of the piezoelectric material used to prevent depolarization.

[0022] One embodiment of the process provides that the aging step follows the heating step, and that the elevated temperature during the aging step is above the solidus line of the metal or metal alloy. In this embodiment, the timing for connecting the piezoelectric discs is determined. The temperature during the aging step is, for example, between 20 °C and 60 °C above the solidus line temperature.

[0023] Furthermore, the invention solves the problem by means of a piezoelectric transducer device manufactured using the method according to one of the preceding or following embodiments. Therefore, the explanations also apply accordingly to the transducer device, so repetition is omitted. The transducer device may also incorporate, for example, contact electrodes, so-called solder lugs, or non-piezoelectric components, such as ceramic insulating discs.

[0024] Furthermore, the invention solves the problem by means of a device for determining and / or monitoring at least one process variable, comprising at least one mechanically vibrating unit, and at least one transducer device for exciting the mechanically vibrating unit to mechanical vibrations and / or for receiving mechanical vibrations from the mechanically vibrating unit. The piezoelectric transducer device is manufactured using the method according to one of the preceding or following embodiments. Therefore, the explanations also apply accordingly to the advantages, so repetition is omitted.

[0025] The invention is explained in more detail with reference to the following figures.

[0026] Fig. 1 schematically shows a device for determining and / or monitoring a process variable. Fig. 2 shows a schematic section through a coated piezoelectric disc according to a first embodiment.

[0027] Fig. 3 shows a schematic section through a coated piezo disk according to a second embodiment.

[0028] Fig. 1 shows the basic structure of a device with which, for example, the density, viscosity or fill level of a medium can be determined.

[0029] The device has a mechanically oscillating unit 101, which is designed here as a vibrating fork and has two fork tines. The transducer device 100 for exciting the mechanically oscillating unit 101 to mechanical vibrations and for receiving the signals from the interaction with the medium (not shown here) has, by way of example, two piezoelectric discs 1, 2. However, there can also be more than two piezoelectric discs.

[0030] The first piezoelectric disk 1 and the second piezoelectric disk 2 are connected to each other via liquid-phase diffusion, in particular by bonding. For electrical contact, preferably at least one contact electrode protruding from the stack of piezoelectric disks 1, 2 – not shown here – is provided.

[0031] Figures 2 and 3 below show variations in how the first piezoelectric disc 1, in particular, can be coated. Other piezoelectric discs may have been processed in a similar manner.

[0032] Figure 2 shows the first piezo disk 1.

[0033] On an end face 10, which faces the second piezo disk 2 in the piezo stack, there is a first layer 11 made of a silver-chromium alloy as an example of a precious metal alloy, a second layer 12 made of tin as an example of a metal that differs from the precious metal, i.e., silver, and is distinguished by a low melting point, and a third layer 13 made of the same silver-chromium alloy as the first layer 11. In the variant shown in Fig. 3, the structure of the variant shown in Fig. 2 with the three layers 11, 12, 13 on the end face 10 of the first piezo disk 1 is supplemented by two further layers. The fourth layer 14 again consists of tin and is protected against oxidation at high temperatures by a fifth layer 15 made of the silver-chromium alloy. In an alternative embodiment, indium is used instead of tin. Reference symbol

[0034] first piezo disk

[0035] second piezo disc

[0036] Front

[0037] first shift

[0038] second layer

[0039] third layer

[0040] fourth shift

[0041] fifth layer converter device

[0042] mechanically oscillating unit

Claims

Patent claims 1. Method for manufacturing a piezoelectric transducer device (100), wherein the method comprises at least the following steps: that on at least one end face (10) of at least one first piezo disk (1) either in a first step a first layer (11) of silver alloy and in a second step on the first layer (11) a second layer (12) of metal or metal alloy, which differs from silver alloy and has a lower melting point than the silver alloy, is applied, wherein a silver chromium alloy is applied as the silver alloy, in that at least the first piezo disk (1) and a second piezo disk (2) are arranged relative to each other such that the end face (10) of the first piezo disk (1) is adjacent to the second piezo disk (2), and in that in a joining step at least the first piezo disk (1) and the second piezo disk (2) are baked together under the influence of pressure and temperature by means of liquid phase diffusion.

2. Method according to claim 1, wherein the first layer (11) and / or the second layer (12) is applied by sputtering.

3. Method according to claim 1 or 2, where tin is applied as the metal or a tin alloy as the metal alloy.

4. Method according to any one of claims 1 to 3, where indium is applied as the metal or an indium alloy as the metal alloy.

5. Method according to claim 4, wherein a tin-indium alloy is applied as the metal alloy.

6. Method according to any one of claims 1 to 5, wherein in a third step a third layer (13) of silver or silver alloy is applied to the second layer (12).

7. Method according to claim 6, wherein in a further step to the third layer (13) metal or A metal alloy - and preferably silver or silver alloy - is applied to it.

8. Method according to any one of claims 1 to 7, wherein in the connection step in a heating step a temperature acting on at least the first piezoelectric element (1) and the second piezoelectric element (2) is maintained for a predetermined period of time above a solidus line of the metal or metal alloy with the lower melting point than the silver alloy.

9. Method according to any one of claims 1 to 8, wherein in the connection step in an aging step at least the first piezoelectric element (1) and the second piezoelectric element (2) are aged by introducing an increased temperature.

10. Method according to claims 8 and 9, where the aging step follows the heating step, and where the increased temperature during the aging step is above the solidus line of the metal or metal alloy.

11. Piezoelectric transducer device (100), manufactured using the method according to any one of claims 1 to 10.

12. Device for determining and / or monitoring at least one process variable, with at least one mechanically oscillating unit (101) and with at least one transducer device (100) according to claim 11 for exciting the mechanically oscillating unit (101) to mechanical oscillations and / or for receiving mechanical oscillations from the mechanically oscillating unit (101).

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